A conventional asynchronous AC electrical motor comprises a rotor and a stator both made of ferromagnetic material of high permeability. The stator, which is the stationary part of the motor, consists of a core and a set of windings. The core is typically constructed using laminated steel and the windings are typically made of copper wire wound around the core. The purpose of the stator is to produce a strong electromagnetic field in which the rotor turns.
The rotor is the part of the motor that is configured to rotate. The rotor is typically positioned inside the stator. The rotor consists of a core, made from magnetic steel, and a shaft. The core is generally pressure fitted onto the shaft. The shaft transmits the mechanical energy converted by the motor from electrical energy to a load.
The stator windings are fed by multi-phase currents to produce a rotating magnetic field. Depending on the geometric layout of the windings and on the current in them, different configurations of the magnetic field in the motor may be produced. Because of Faraday's law, the stator magnetic field causes an EMF to be induced on the rotor which generates current in the rotor. This current interacts with the magnetic field and produces a torque that rotates the rotor.
A common two-phase motor consists of two sets of windings on the stator. The first set is the main winding or the run winding. The second set is the auxiliary or start winding. The two windings are connected in parallel with each other.
When the motor is initially energized, voltage is applied across both windings. In order to create a rotating magnetic field, the phase of the applied voltage on the start winding must be shifted from the voltage on the run winding. This can be done by placing a capacitor in series with the start winding. The phase shift can also be created electronically.
In many applications, it is important to accurately determine and control the rotational speed of the rotor in an electric motor. One example in which this is important is the use of an electric motor to rotate the anode of an xray tube to produce a high power output x-ray tube.
Modern rotating anode x-ray tubes include a rotor, positioned in the vacuum inside of the glass x-ray tube envelope, and a stator positioned outside the glass envelope. The rotor shaft is connected to an x-ray tube anode, which is typically a large disc of tungsten or an alloy of tungsten, causing the anode to rotate with the motor rotor when an x-ray exposure is being made.
The ability of the x-ray tube to achieve desirable high power x-ray outputs is limited by the heat generated on the anode in the x-ray tube. The purpose of the rotating anode is to spread the heat produced during an exposure over a large area of the anode. Thus by rotating the anode, it has been possible to produce x-ray tubes capable of withstanding the substantial heat generated by high power x-ray tube outputs.
In a typical rotating anode x-ray tube, the standard number of revolutions of the anode is approximately 3,600 RPM for normal exposures and 10,000 RPM for high intensity exposures. The speed is set by predetermining a motor input signal which is expected to achieve the desired speed, applying the predetermined motor input to the motor prior to making an x-ray exposure and waiting a predetermined amount of time for the rotor to get up to speed. In practice, the anode never reaches a speed of 3,600 RPM or 10,000 RPM because of mechanical factors such as slipping between the rotor and the bearings. Thus, a speed correction (usually -10% to -20%) is generally assumed when calculating the predetermined motor input.
Typically, in order to maintain a rotational speed of 3,600 RPM a 60 VAC motor input signal is provided and in order to maintain a rotational speed of 10,000 RPM a motor input signal of 100 VAC is provided. The normal acceleration time for a 3" anode is around 2 seconds. The normal acceleration time for a 4" anode is approximately 2.5-3.0 seconds. During the time required to bring the rotor up to speed, the system is "on hold" and radiographic exposure is not allowed.
The extremely adverse conditions created by high power x-ray exposures make measuring the actual speed of rotation of the anode very difficult. Furthermore, motors used for rotating the anode in a rotating anode x-ray tube typically have a very low efficiency due to the large vacuum, air and glass gap between the rotor and stator winding. Thus, conventional methods of measuring the rotational speed of a motor rotor are typically not effective for measuring the speed of rotation of the anode in a rotating anode x-ray tube.
Current x-ray apparatus often make the assumption that if they provide the predetermined motor input signal and wait the predetermined amount of time then the anode will be rotating at the appropriate speed. These systems do not actually measure the speed of rotation of the anode. Thus, the possibility exists that if the circuitry or motor fails, exposure could be made on a stationary anode destroying the x-ray tube.
Additionally, in most cases, the anode rotational speed does not need to be at its maximum in order for exposure to begin. By allowing exposure at speeds lower than the maximum, valuable time can be saved and the optimum parameters can be achieved. However, prior art systems that do not actually measure the rotational speed of the anode cannot take advantage of these efficiencies.
Prior art devices that do measure the speed of rotation of the anode do so indirectly, such as by using a vibration tachometer. However, these methods typically are complicated, require expensive equipment and are not very accurate. Furthermore, vibration tachometers typically only indicate when a rotor is rotating at a predetermined speed. They do not provide any information regarding the rotor speed if the rotor is rotating at an intermediate speed. Thus, it is apparent that there is a need for an improved method of measuring the speed of rotation of an AC motor rotor.
An object of the present invention is to provide a simple and accurate method of measuring the actual speed of rotation of the rotor in an AC motor.
Another object of the present invention is to provide a method and apparatus for accurately measuring the actual speed of rotation of an anode in a rotating anode x-ray tube.